1. Field of the Invention
This invention relates to the hydraulic fracturing of subterranean formations surrounding oilwells, gas wells, water wells, and similar boreholes. In one aspect it relates to multiple fracturing of such formations.
2. Description of the Prior Art
Hydraulic fracturing is a well stimulation technique which involves injecting a fracturing fluid into the formation at rates and pressures sufficient to rupture the formation. In most formations, the earth stresses are such that a vertical crack (fracture) is formed by the hydraulic fracturing treatment. Once the fracture is initiated, continued injection of the fracturing fluid causes the fracture to grow in length and width. A particulate material suspended in a carrier fluid is then placed in the fracture to maintain it in a propped condition when the fracture-inducing pressure is relieved.
In the fracturing of most formations, it is desirable to maximize the width of the propped fracture. It is known that well stimulation is strongly influenced by the width of the propped fracture. When the product of width and fracture permeability is too small, fracture lengths will not improve well stimulation. However, as the fracture width increases, increased fracture lengths can improve well stimulation. The maximum fracture width is normally obtained by controlling variables, such as fluid viscosity and injection rate to achieve the desired fracture geometry. Although large dynamic widths are frequently obtained, the width of the closed fracture is substantially less than the dynamic width, mainly because of the relatively low concentration of proppant in the carrier fluid. In other words, most of the volume in the carrier fluid is liquid, which leaks off into the formation, leaving the proppant wedged between the formations walls.
Although the importance of fracture width has long been realized, widths larger than about 0.25 inches have probably not been achieved.